The present invention relates to a method for manufacturing a high Cr system seamless steel pipe, which is preferably employed as a structural material for constructing an oil well, a gas well, one of various plants or the like, and more specifically to a method for manufacturing a high Cr system seamless pipe, which ensures a reduced rate of generating the inside surface defects thereof, even if a seamless pipe is manufactured from a primary material (billet) for producing the pipe, which includes a Cr content of 10 to 20%.
Conventionally, a so-called high Cr system seamless steel pipe, which includes a Cr content of 10 to 20%, has been widely employed as a structural material for constructing an oil well, one of various plants or the like. Such a seamless steel pipe is produced in the following steps: Firstly, a hollow primary pipe is formed from a round bloom with the Mannesmann piercing process, the press piercing process or the like, and secondly, using a stretching mill, such as a mandrel mill, plug mill or the like, the hollow primary pipe is further shaped to increase the diameter thereof and at the same time to reduce the wall thickness thereof, and thereafter further shaped to form a finished pipe having a desired size, using a reducing mill, such as a stretch reducer.
In the case of manufacturing the above-mentioned high Cr system seamless steel pipe, a round billet, which is produced by rolling an ingot manufactured by the continuous casting process or the ingot blooming process, is used as a primary material (billet) for producing the pipe. Typically, the billet used as such a primary material is manufactured in the following steps: An ingot (bloom) having a rectangular cross section is formed by the continuous casting process or the ingot blooming process, and, after uniformly heated over a wide area at a predetermined temperature, the bloom is either hot-rolled into a round shape with a stabbing mill, blooming mill, or the like, or continuously cast into a round bloom.
The high Cr steel is normally inferior to the conventional steels, regarding the hot workability and therefore defects often generate on the inside surface of the steel pipe after the pipe is produced. When, for instance, defects such as inside small scabs (hereinafter referred to as xe2x80x9cthe inside surface defectsxe2x80x9d) are generated on the inside surface of the steel pipe, not only the yield in the production of the pipes is decreased, but also the mill train including a stretching mill and a reducing mill, along with a piercing mill, has to be stopped. Accordingly, the productive efficiency in the total system is greatly reduced.
In order to avoid the generation of such inside surface defects in the production of seamless steal pipes with the hot working, several means have been usually employed, in which, for instance, either the degree of working in the course of producing the pipes is reduced or the temperature at which the primary material is processed is decreased to reduce the number of defects generated by heating due to the working. However, the above-mentioned means cause the productive efficiency in producing the pipes with the hot working to decrease, and therefore it cannot be stated that these means are appropriate for suppressing the inside surface defects.
In Japanese Patent Application Laid-open No. 04-224659, for instance, a method for manufacturing a martensitic seamless steel pipe is proposed, in which the texture may be improved in the hot working by the contents of several alloy elements within certain ranges, and by controlling the period of annealing, together with the usage of a relative low temperature of not more than 1,200xc2x0 C. in the piercing treatment. In this method, however, the type of steels applicable thereto is restricted because the control of the specified elements in the alloy is severe, and at the same time, the restriction of the upper limit in the heating temperature for forming the pipe with the piercing process provides not only a reduction in the productive efficiency of the pipe as well as in the productivity of the total system, but also a decrease in the service time of tools used for manufacturing the pipe.
As described above, the conventional means for suppressing the inside surface defects, which means is employed in the manufacturing the pipe using such a hard-workable material as high Cr steel or the like, have required a reduction in the degree of working as well as in the heating temperature. This inevitably has provided a reduction in the productivity for manufacturing the pipe, thereby making it difficult to enhance the productive efficiency of the total system.
Taking into account these problems in the prior art, it is an object of the present invention to provide a method for manufacturing a seamless steel pipe of a high Cr system, which method ensures to effectively prevents the inside surface defects from generating without any reduction in the productivity, when manufacturing a seamless steel pipe from a bloom or billet of a high Cr steel system as a primary material for producing the pipe.
The generation of the inside surface defects in manufacturing a high Cr system seamless steel pipe results from the crack generation at fragile parts of the texture due to the stress in the work of producing the pipe, and from the further development of the cracks to the inside surface defects, because the hot workability of such a steel is inferior. The fragile parts in a hot-worked high Cr steel are grain boundaries between austenite xcex3 particles and xcex4 particles, where the austenite xcex3 particle is one of the main textures at a high temperature of the steel and the xcex4 particle is included at a very small amount together with the generation of xcex4 ferrites.
Accordingly, in order to reduce the inside surface defects generated in the hot working, [1] it is necessary to reduce the fragile parts in the textures by decreasing the number of the generated xcex4 ferrites, and [2] it is necessary to increase the mechanical strength of each grain boundary between an austenite xcex3 particle and a xcex4 particle. As the first means [1], the reduction of the amount of the impurity elements (S and P), which make the grain boundaries fragile, is effective, but an excessive reduction causes the manufacturing cost to increase. On the other hand, the method proposed by the above-mentioned Japanese Patent Application Laid-open No. 04-224659 is effective as the second means [2]. However, in order to enhance the productive efficiency in manufacturing the seamless steel pipe, a further improvement is required to the practical applications.
After detailed investigations, the present inventors have found that the degree of influence of alloy elements and Cr contained on the generation of xcex4 ferrites can be quantitatively expressed and the degree of the influence of the thermal history in the stage of manufacturing the billet and in the pre-stage of manufacturing the pipe from the primary material on the amount of xcex4 ferrites generated can also be quantitatively expressed.
By further applying the obtained results to the actual production lines, the present inventors have found that an inexpensive seamless steel pipe having an excellent inside surface quality can be produced with a high productive efficiency, even if the amount of impurity elements (S and P) is excessively reduced, and even if the pipe manufacturing conditions are further moderated.
The present invention is accomplished on the basis of the above-described findings, and thus provides the following two methods (1) and (2) for producing a high Cr system seamless steel pipe:
(1) A method for manufacturing a high Cr system seamless steel pipe, wherein an initial material including Cr at a content of 10 to 20 mass %, impurities S and P at respective contents of not more than 0.050 mass %, and one or more of C, Mn, Ni, N, Cu, Si, Mo, Ti, Nb and V is heated for soaking at a temperature of not less than 1,100xc2x0 C. for a total soaking period xcexa3t1 (hours) to form a primary pipe material as a billet or bloom, and thereafter the primary pipe material is further heated for soaking at a temperature of not less than 1,100xc2x0 C. for a total soaking period xcexa3t2 (hours) and then heated at a temperature of 1,200xc2x0 C. to form a finished pipe, wherein the soaking and/or the heating is carried out so as to fulfill the following equation (b),
f={20xc3x97C+0.3xc3x97Mn+1.2xc3x97Ni+25xc3x97N+Cuxe2x88x929xc3x97Sixe2x88x920.8xc3x97Crxe2x88x922xc3x97Moxe2x88x9210xc3x97Tixe2x88x926xc3x97Nbxe2x88x9215xc3x97V}xe2x88x9245xc3x97(S+P/10)xe2x80x83xe2x80x83(a)
                    F        =                              f            +                          0.6              xc3x97                              (                                  1                  -                                      1                                          e                                              ∑                                                  xe2x80x83                                                ⁢                        t1                                                                                            )                                      +                          0.8              xc3x97                              (                                  1                  -                                      1                                          e                                              ∑                        t2                                                                                            )                                               greater than                       -            9.7                                              (        b        )            
where element symbols in the equation (a) represent the contents of the corresponding elements (mass %).
(2) A method for manufacturing a high Cr system seamless steel pipe, wherein an initial material including Cr at a content of 10 to 20 mass %, impurities S and P at respective contents of not more than 0.050 mass %, and one or more of C, Mn, Ni, N, Cu, Si, Mo, Ti, Nb and V is heated for soaking at a temperature of not less than 1,100xc2x0 C. for a total soaking period xcexa3t1 (hours) to form a primary pipe material as a billet or bloom, and thereafter the primary pipe material is further heated for soaking at a temperature of not less than 1,100xc2x0 C. for a total soaking period xcexa3t2 (hours), and thereafter, the primary pipe is heated at a temperature of 1,100 to 1,300xc2x0 C. (except for 1,200xc2x0 C.) to form a finished pipe, wherein the soaking and/or the heating is carried out so as to fulfill the following equation (c),                     F        =                              f            +                          0.6              xc3x97                              (                                  1                  -                                      1                                          e                                              ∑                                                  xe2x80x83                                                ⁢                        t1                                                                                            )                                      +                          0.8              xc3x97                              (                                  1                  -                                      1                                          e                                              ∑                        t2                                                                                            )                                      +                          1.4              xc3x97              KT                                 greater than                       -            9.7                                              (        c        )            
where   KT  =                    1200        -        T                              "LeftBracketingBar"                      1200            -            T                    "RightBracketingBar"                      .  